1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus using the same, in particular, relating to a fixing device that fixes an unfixed toner image to a recording medium by heating the unfixed toner image formed on the recording medium as well as to an image forming apparatus using the same.
2. Description of the Prior Art
Conventionally, an electrophotographic image forming apparatus (which will be referred to simply as ‘image forming apparatus’ hereinbelow) includes: for example, a photoreceptor, a charging means, an exposure means, a developing means, a transfer means and a fixing means.
The image forming apparatus is a machine that performs a charging process, exposure process, developing process, transfer process and fixing process by use of the photoreceptor and these devices to form an image on a sheet-like recording medium (which will also be referred to simply as ‘sheet’ hereinbelow). As a fixing means for performing the fixing process, for example a heat roller type fixing device is used. The heat roller type fixing device includes a fixing roller and a pressing roller. The fixing roller and the pressing roller are a pair of rollers that are put in pressing contact with each other. At least one of the fixing roller and pressing roller incorporates a heat source such as a halogen heater or the like as a heating means.
In the fixing process, the roller pair is heated by the heat source to a predetermined temperature (which will be referred to hereinbelow as ‘fixing temperature’) necessary for fixing, then a recording medium with an unfixed toner image formed thereon is delivered to a fixing nip portion, or the pressing contact part, between the fixing roller and the pressing roller.
The unfixed toner image passing through the fixing nip portion is fixed to the recording medium such as paper or the like, with the heat transferred from at least one of the fixing roller and the pressing roller and under the pressure of the fixing roller and the pressing roller. The area, in the fixing nip portion, through which the recording medium has passed (which will be referred to hereinbelow as ‘sheet passing area’) lowers in temperature, but is heated to the fixing temperature by means of the heat source.
In a fixing device provided for an image forming apparatus capable of performing full-color printing, a fixing roller having an elastic layer made of silicone rubber or the like, formed on the surface thereof (which will be referred to hereinbelow as ‘elastic roller’) is used. Use of the elastic roller enables the elastic layer on the elastic roller surface to elastically deform in the fixing nip portion in conformity with the ruggedness of the unfixed toner image so that the elastic roller embraces, covers and comes into contact with, the unfixed toner image. Accordingly, this configuration exhibits more improved fixing performance for a color unfixed toner image which carries a greater amount of toner than a mono-color image.
Further, due to the strain releasing effect of the elastic layer on the elastic roller surface, it is possible to improve separation performance of the color toner which is more likely to offset compared to a mono-color image. More specifically, the elastic layer, which has been compressed and deformed in the fixing nip portion, is freed from its deformed condition at the exit of the fixing nip portion, so that a slippage arises between the elastic layer and the toner image at the exit of the fixing nip portion. As a result, the adherence of the elastic layer to the toner image lowers, hence the separation performance improves.
Further, since the nip configuration or the form of the fixing roller and the pressing roller in the fixing nip portion, is projected to the fixing roller side (forming an inverted nip configuration), it is possible to improve separation performance between the fixing roller and the recording medium. Accordingly, without use of any separation claws or the like as a separating means for separating the recording medium from the fixing roller, it is possible to realize a self-stripping mechanism that enables the recording medium to separate from the fixing roller and hence it is possible to dispel image defects attributed to separating means.
In order to make the fixing device deal with high-speed printing, it is necessary to make the width of the fixing nip portion (which will be referred to hereinbelow as ‘fixing nip width’) greater. As a means for making the fixing nip width greater, two methods can be considered; one way is to make the elastic layer of the elastic roller thicker, and the other is to increase the elastic roller in diameter.
However, since the elastic layer of the elastic roller is markedly low in thermal conductivity, when a heating device is provided inside the elastic roller as in the prior art elastic roller there occurs the problem that it takes long time for warming up if the elastic layer of the elastic roller is thick. Also, there occurs a problem that the temperature of the fixing roller cannot follow the fixing temperature when the processing speed is made higher.
Further, there occurs another problem that power consumption of the heating means increases as the elastic roller is made greater in diameter.
In order to solve these problems, as a prior art technology there has been a proposal of a fixing device based on a belt fixing mechanism including a fixing roller, a pressing roller, a heat roller and an endless belt, wherein an endless belt is wound between the fixing roller and the heat roller incorporating a heater for heating while the fixing roller and pressing roller are put in pressing contact with each other with the endless belt disposed therebetween (see patent document 1: International Publication WO99/00713).
According to the fixing device disclosed in patent document 1 (International Publication WO99/00713), it is possible to reduce the warm-up time because the endless belt having a small heat capacity is heated by the heat roller as a heating means without the necessity of heating the elastic layer having a greater heat capacity. Further, since it is possible to provide a thick elastic layer having a low hardness such as sponge rubber or the like without the necessity of heating means built in the fixing roller, it is possible to assure a large fixing nip width.
As another technology, there has been a proposal of a fixing device (see patent document 2: Japanese Patent Application Laid-open Hei5-No. 289556), in which a heating means given in the form of a planar heat generator is used to apply heat to the recording medium as a heated material through a film member so as to heat and fuse the unfixed toner image on the recording medium and thereby fix the image to the recording medium.
According to the fixing device disclosed in patent document 2 (Japanese Patent Application Laid-open Hei5-No. 289556), since the planar heat generator has a lower heat capacity than halogen lamp heaters, it is possible to suppress the heat capacity of the heating means compared to the prior art configuration with a halogen lamp heater. It is therefore possible to reduce power and shorten the warm-up time.
However, in the technology described in patent document 2, the heater portion takes a form in which a ceramic heater is directly bonded to a heat transfer plate. As a result, this configuration entails the problem that the ceramic heater is broken by the stress arising in the bonded area between the heater and the heat transfer plate due to frictional movement of the paper and the problem that heat transfer cannot be done satisfactorily due to change of the contact condition between the ceramic heater and the transfer plate.